Conventionally, as a filtration tank capable of treating sewage flowing into a sewage treatment plant at a high speed, an upflow filtration tank having a floating filter media layer formed of floating filter media made of foamed resin and an upper screen disposed over the floating filter media layer and supporting the floating filter media is known (see, for example, Patent Literature 1).
In the upflow filtration tank, sewage is passed upward through the floating filter media layer and the suspended solids (SS) in the sewage are captured by the floating filter media layer, thus the sewage is filtered and filtrate water from which suspended solids have been removed can be obtained. Further, in the upflow filtration tank, due to the captured suspended solids, a pressure loss is increased in the floating filter media layer during passage of sewage, and when the filtration tank cannot provide a desired filtration performance, backwash water is passed downward through the floating filter media layer and the captured suspended solids are discharged by spreading the floating filter media downward, thus the floating filter media layer is backwashed (i.e. cleaned by back-flow) and a filtration performance of the filtration tank can be recovered.
In the above-described conventional upflow filtration tank, backwash water flow can concentrate only on a portion where water can flow easily (a water path is formed) during backwashing. Further, when only the portion where a water path has been formed during backwashing is washed, a portion where a relatively large amount of suspended solids have been captured (heavily clogged portion) and a portion where a relatively small amount of suspended solids have been captured (lightly clogged portion) can be produced in the floating filter media layer. Then, once both the heavily clogged portion and the lightly clogged portion are produced in the floating filter media layer, flow of backwash water concentrates only on the lightly clogged portion where water can flow easily during backwashing. Thus a floating filter media does not uniformly spread across the floating filter media layer, and the floating filter media layer cannot be thoroughly and uniformly backwashed. Further, when flow of backwash water concentrates on the lightly clogged portion, flow rate of the backwash water on the lightly clogged portion is increased, which causes the floating filter media to spread excessively downward, and to flow out from the outlet of backwash water disposed in the lower part of the filtration tank.
Thus, as a method of washing by distributing backwash water uniformly across the entire floating filter media layer during backwashing and is by spreading floating filter media uniformly, it has been suggested that the percentage of the aperture (aperture ratio=(aperture area/upper screen area) ×100) provided on the upper screen of the filtration tank be a certain value within a range of 1 to 30% (see, for example, Patent Literature 2).